1-Butanol pervaporation performance and intrinsic stability of phosphonium and ammonium ionic liquid-based supported liquid membranes

Journal of Membrane Science 429 (2013) 214–224

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1-Butanol pervaporation performance and intrinsic stability of phosphonium and ammonium ionic liquid-based supported liquid membranes Hercules R. Cascon a,n, Santosh K. Choudhari a,1 a

Department of Environmental Engineering and Biotechnology, Myongji University, San 38-2 Namdong Yongin-Si, Gyeonggi-do 449-728, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 May 2012 Received in revised form 31 October 2012 Accepted 2 November 2012 Available online 23 November 2012

The intrinsic stabilities of simple supported liquid membranes and their pervaporative recoveries of 1-butanol from dilute aqueous solutions were investigated. Hydrophobic ammonium- and phosphoniumbased room temperature ionic liquids were used as the liquid membranes. The membranes performed better than or comparably to other pervaporation membranes. 1-Butanol flux was highly positively correlated with the ionic liquid’s partition coefficient for 1-butanol and was inversely correlated with the membrane’s hydrophobicity and viscosity. Water flux was strongly influenced by the ionic liquid’s water saturation capacity. Except at the highest temperature investigated (70 1C), no trade-off was seen between separation factor and temperature. Diffusivity and activation energy results suggested the presence of water microenvironments in the membranes, which influenced permeant transport. Permeances and membrane selectivities indicated that transport was dominated by sorption rather than diffusion. Membranes’ selectivities consistently increased with increasing feed concentration. Sustained pervaporation for  90 h showed that the ionic liquid required a minimum level of hydrophobicity to produce a stable membrane. Diluting the ionic liquid with oleyl alcohol enhanced separation by increasing the membrane’s partition coefficient for 1-butanol and decreasing its viscosity; albeit temporarily as the fatty alcohol was gradually leached during sustained testing. & 2012 Elsevier B.V. All rights reserved.

Keywords: Butanol Pervaporation Ionic liquids Supported ionic liquid membrane (SILM) Liquid membrane blend SILM stability

1. Introduction Butanol is a potentially useful biofuel, though its production suffers from high separation costs associated with its very low concentrations attained during fermentation. Therefore, its cost effective recovery from dilute fermentation broths is required. Membrane-based separation via pervaporation is promising for such extractions as its operation is at low temperature and pressure and it is particularly effective with low feed concentrations of the permeating species [1]. A promising alternative to polymeric dense or mixed-matrix membranes is supported liquid membranes that incorporate a liquid extractant immobilized inside the pores of a support matrix. Molecular diffusion in liquids is faster by several orders of magnitude than in solids, allowing supported liquid membranes to show higher selectivities and fluxes. Selectivity can be achieved not only towards particular groups of substances, but even for individual ions or molecules [2].

n Corresponding author. Current address: Department of Chemical Engineering, Xavier University-Ateneo de Cagayan, Cagayan de Oro, Misamis Oriental 9000, Philippines. Tel.: þ 63 9052805186; fax: þ 63 88 858 3116. E-mail addresses: [email protected] (H.R. Cascon), [email protected] (S.K. Choudhari). 1 Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College, Dublin 2, Ireland.

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.11.028

Hydrophobic organic solvents with high 1-butanol affinity have been used as liquid membranes (LMs) for 1-butanol pervaporation, though their use has not been fully explored [3,4]. Supported ionic liquid membranes (SILMs) in their simplest configuration comprise a support membrane with pores impregnated with a room temperature ionic liquid (RTIL). RTILs are novel alternatives to organic solvents due to their negligible vapor pressures, wide liquid ranges, and high extraction capacities [5,6]. RTILs can be selected or designed to have properties tailored to particular applications; they have hence been dubbed designer solvents [5]. RTILs have not been widely studied for use in butanol pervaporation. Iza´k et al. [7,8] reported ionic liquid-polydimethylsiloxane blends that showed improved membrane separation and stability. However, the stabilities of pure ionic liquids and their use in pervaporative separation have not been reported. In addition, few of the currently available RTILs have been investigated for such applications. Imidazolium-based ionic liquids have been studied extensively, though phosphonium- and ammonium-based RTILs are gaining increasing attention as they are less expensive, more easily synthesized [9] and show higher thermal and chemical stabilities, favorable viscosity, hydrophobicity, and solute extraction capacities [9,10]. Representative RTILs such as trihexyl(tetradecyl)phosphonium bis (trifluoromethylsulfonyl)imide ([Ph3t][NTf2]) and trihexyl(tetradecyl) phosphonium dicyanamide ([Ph3t][DCN]) are either nontoxic or only

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inhibitory to solventogenic bacteria Clostridium beijerinckii at saturation concentrations in fermentation broths [11]. These advantageous properties make RTILs suitable LM extractants. High thermal and chemical stabilities are necessary for sustained high-temperature applications. Hydrophobicity is important to SILMs’ stability and may also affect the efficiency of partitioning the target solute from the aqueous phase, together with the LM’s solute extraction capacity. Viscosity greatly affects the diffusivities of the permeating species across the LM extractant phase [6]. RTILs’ biocompatibility with fermentation microorganisms allows SILMs to be integrated into fermentation–pervaporation systems. This work reports phosphonium- and ammonium-based RTILs as LM extractants in SILMs for the recovery of 1-butanol by pervaporation. The SILMs’ intrinsic stabilities were assessed during sustained pervaporation. For this work, intrinsic stability is defined as the inherent trait of unmodified (i.e., simple) SILM to give constant selectivity and permeability during its long-term use. Blending the LM to fine tune transport properties such as viscosity and extraction capacity so as to improve pervaporation performance was also explored. Basic SILMs, in which RTIL was immobilized in the support matrix solely by capillary forces, were chosen for their simplicity, which would facilitate analysis of ionic liquid-mediated transport. This work aimed not to develop a stable SILM but rather to provide a fundamental investigation of RTILs’ applicability as selective components in membranes, making it a basis for future work that aims to design more advanced ionic liquid-based membranes.

215

physical properties such as density (r), viscosity (m), surface tension (d), and solubility data are given in Fig. 1 and Table 1. 1-Butanol for partitioning and use as a GC standard was 99.8% HPLC grade (Aldrich). 1-Butanol for the pervaporation feed was 99.0% minimum purity (Showa, Japan). The support matrix, Celgards 2400, was generously provided by Celgards (North Carolina, USA). It is a hydrophobic, stretched type microporous poly(propylene) flat sheet membrane, with average pore dimensions of 0.117  0.042 mm2, 25 mm thickness, 37% porosity (e), and 2.70 tortuousity (t) (calculated through t ¼1/e) [17]. The membrane was compatible with all the tested LM extractants and showed favorable thickness and tortuousity. It has been used in other supported liquid membrane studies, allowing for useful comparison. 2.2. Determination of the LM extractants’ 1-butanol partition coefficients The LMs’ extraction capacities for 1-butanol were determined through measuring partition coefficients (KP) using a method described elsewhere [11]. The coefficients were calculated using the following equation:   C OP ð1Þ KP ¼ C AP eq where C denotes 1-butanol concentration. Subscripts OP and AP respectively denote the organic and aqueous phases and eq denotes equilibrium. Equilibration temperatures varied from 25 to 70 1C.

2. Experimental 2.3. SILM preparation

2.1. Materials Three RTILs, [Ph3t][NTf2] ( 495%), trioctylmethylammonium bis(trifluoromethylsulfonyl)imide [OMA][NTf2] ( Z99%), and [Ph3t][DCN] ( Z95%), were from Aldrich (Missouri, USA). Oleyl alcohol (OA, technical grade, purity ca. 60%, Acros Organics, Geel, Belgium) was used to dilute the RTILs since it is miscible with the RTILs. All were used as received. LM extractants’ structures and

SILMs were prepared by impregnating the LM extractant (pure or blended) into circular (57 mm diameter) support matrices. Ca. 0.2 mL LM extractant for every four supports was spread in a Petri dish and then each support was carefully placed over the LM extractant avoiding air entrapment. The LM was absorbed quickly and the supports were flipped to allow thorough absorption. Soaking was for overnight under vacuum to drive off any entrained gases. Excess liquid was removed from the support matrices by gentle blotting with fine-grained filter paper and wiping with lint-free tissue paper. The supports’ LM capacities (LMC) were measured gravimetrically by weighing the membranes before and after LM immobilization. 2.4. Membrane characterization 2.4.1. FESEM characterization The morphologies of the pristine support membrane and the fresh and used SILMs were characterized by field emission scanning electron microscopy (FESEM EDS, JEOL JSM 6700F Japan)

Fig. 1. RTILs used as LMs in this study.

Table 1 RTILs used as LM in the study. Abbreviated name

[Ph3t][NTF] [OMA][NTF] [Ph3t][DCN] a

Molecular formula

C34H68F6NO4PS2 C27H54F6N2O4S2 C34H68N3P

MW

764.0 648.8 549.9

Mole fraction solubility of water in ionic liquid extractant. Density data from product literature. c Measured gravimetrically in this study. d Solubility of extractant in water (solubility of water in extractant). b

Physical properties (at 25 1C)

r (g/cm3)

m (cP)

d (dyne/cm)

Solubility data

1.080 [12] 1.105b 0.904 [12]

145 [12] 136 [14] 201 [12]

33.08 [13] 27.93 [15] 35.04 [13]

0.087a[10] 0.177a,c 0.510a [10]; 72.7 g/m3d [16]

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operated at 5.0 kV acceleration voltage, with images scanned at 5000  magnification.

2.4.2. Contact angle measurement Water contact angles were measured to assess the SILMs’ hydrophobicity. Rectangular samples (0.5 cm  1.5 cm) of pristine support and the freshly prepared and used SILMs were mounted on clean glass slides. A GC injector syringe was used to drop deionized water (2 mL) at random points on the membrane strips. The axisymmetric drops were observed using a microscope coupled to a CCD camera (Video Loupe VL-11S, Japan) and both contact angles of each image were measured using a protractor. At least five distinct droplet images were analyzed for each sample; the contact angle measurements were averaged and standard deviations were calculated.

2.5. 1-Butanol recovery by pervaporation The prepared SILMs were tested for the recovery of 1-butanol by standard pervaporation (Fig. 2). For each run, a freshly prepared SILM was mounted in a circular permeation cell supported on a porous sintered steel disk (area¼17.35 cm2). A 300 mL/min feed flow was sufficient to minimize concentration polarization and the retentate was recycled to the feed tank. The temperature of the feed was varied from 35 to 70 1C, controlled to an accuracy of 72 1C. A vacuum pump provided a driving force through a partial vacuum of r0.31 kPa on the downstream side of the SILM. Permeate vapors were condensed in a liquid nitrogen cold trap. Pervaporation was held at equilibrium for at least 1 h, with the collected permeate returned to feed tank, prior to collection of the final permeate samples to ensure the attainment of a steady-state. At least 2 l of feed solution, with butanol concentrations of 0.5–2.5 wt%, was used. Depending on the SILM, pervaporation runs were for 0.5–5 h to ensure no significant LM loss or change in feed concentration. 1-Butanol concentrations in the feed and permeate were analyzed through gas chromatography (Section 2.7). Each set of conditions was tested at least three times. The SILMs’ intrinsic stabilities were assessed by subjecting them to sustained pervaporation. The feed solution and permeate were periodically sampled and their concentrations of 1-butanol and dissolved RTIL were measured (Section 2.7). The feed’s concentration of 1-butanol was maintained by returning permeate collected during equilibration periods to the feed tank, using a sufficiently great feed solution volume compared with permeate removed, and adding 1-butanol to the feed tank as necessary.

2.6. Parameters Pervaporation performance parameters were calculated using the following equations:    Y 1X Separation factor, b1BuOH=water ¼ ð2Þ 1Y X Component mass flux,J i ¼

mi ðeAÞt

Pervaporation separation index,PSI ¼ J T ðb1Þ

ð3Þ ð4Þ

where X and Y are the molar fractions of butanol in the feed and permeate, respectively; m refers to the mass (g) of component i (or j) collected during t hours of pervaporation, and JT is the total flux in g/m2h. A (m2) is the exposed surface area and e is the porosity of the support matrix. The active mass transfer area (eA) represents the transport characteristics of the LM extractant. Baker et al. reported that interpretations of pervaporation data solely based on Eqs. (2)–(4) could be limited since flux and separation factor vary with the effects temperature and feed concentration have on vapor–liquid equilibrium [18]. More meaningful insight into a membrane’s permeation properties can be obtained through analyses of permeance and selectivity Component permeance,

SILM selectivity, aij ¼

Pi J ni ¼ l gio xio psat pil io

Pi =l P j =l

ð5Þ

ð6Þ

where Jn, g, x, psat, and p are the molar flux (with units cm3(STP)/ cm2s, mass transfer area taken as eA), activity coefficient, molar fraction, saturated vapor pressure at the feed side, and the permeate side pressure of component i or j, respectively. Subscripts o and l refer to the feed and permeate membrane surfaces, respectively. Permeance was selected over permeability to normalize any possible decrease in immobilized LM thickness during pervaporation. The activity coefficients of 1-butanol (component 1) and water (component 2) were estimated using the van Laar system of equations ln g1 ¼ 

A012 A0

1 þ A012 xx12

2

ð7Þ

2

ð8Þ

21

ln g2 ¼ 

A021 A0

x2 1 þ A21 0 x1 12

For any temperature within 25–100 1C range, the corresponding van Laar coefficients (A’12 and A’21) were computed using published limiting activity coefficients of 1-butanol and water in 0 dilute systems according to the relationships ln g1 1 ¼ A12 and 0 sat 1 ln g2 ¼ A21 [19,20]. Values of p (in mm Hg) for 1-butanol and water in the feed were estimated using the Wagner vapor pressure equation [21] for 1-butanol and the Antoine equation for water [20]. Diffusion coefficients (or diffusivities) were estimated based on the methodology proposed by Cao and Henson [22] for nonporous pervaporation membranes. By Fick’s first law   Di dC i
Ji ¼  ð9Þ dz 1ji

Fig. 2. Schematic diagram of the laboratory-scale pervaporation setup.

the flux of component i can be related to its diffusion coefficient (D), its volume fraction (j) within the LM phase, and its change of concentration (Ci) along the flux direction (z). The solution-diffusion mechanism is assumed to apply in the LM1-butanol–water ternary system where the permeants may possibly

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interact with each other and with the LM. Permeating species’ diffusion coefficients are generally affected by temperature, their concentrations, and coupling effects [22]. At constant temperature, Ci can be expressed as the product of a component’s density (ri) and volume fraction, leading to the dependence of Di on the concentration of each component in the liquid mixture. Therefore   Di ri d ji
Ji ¼  ð10Þ dz 1ji

ji would range from a maximum (j0i ) at the feed side (z ¼0) to zero at the permeate side (z¼l). The constancy of D can be verified through the integrated form of Eq. (10):   1 o
Ji l ¼ Di ri ln 1fi ð11Þ

t

where the SILM thickness, l, was corrected with the support matrix’s tortousity, t. At different feed concentrations, the corresponding j0i (or j0j ) can be calculated using KP and the physical data of the RTIL, 1-butanol, and water. Eq. (11) suggests that if Di (or Dj) is constant and not a function of concentration, a plot of Jil versus ln(1  j0i ) (or Jjl versus ln(1  j0j )) would be linear; otherwise alternative, nonconstant assumptions for D should be used [22]. 2.7. Analytical methods A Hewlett Packard 6890 Series GC with HP-FFAP column and flame ionization detector was used for the GC analysis of the samples containing 1-butanol which were appropriately diluted with deionized water. The column was programmed to heat initially to 60 1C and hold for 4 min, ramp to 100 1C at 6 1C/min with zero holding, and then ramped further to 200 1C at 8 1C/min. The inlet and detector were 300 and 250 1C, respectively. Nitrogen was used as a carrier gas at a flow rate of 30 mL/min at a split injection ratio of 60:1. Isopropanol was used as an internal standard. The concentration of phosphonium-based RTIL in the feed was measured through total phosphorus (TP) analysis using a Seal Analytical AutoAnalyzer 3 (AA3). This employed a continuous flow system of feed sample digestion using alkaline persulfate and sulfuric acid reagents at high temperature (110 1C) and pressure (0.13 MPa), followed by a colorimetric reaction with molybdate and ascorbic acid to form a blue phosphor–molybdenum complex that was then read spectrophotometrically at 800 nm [23]. The obtained TP readings were then stoichiometrically converted to RTIL concentrations (ppm).

Fig. 3. 1-Butanol Kp values of pure LM extractants with respect to temperature. Error bars denote standard deviations from three measurements.

due to their respective cations. [OMA] has shown a slightly higher capacity for hydrogen boding than [Ph3t] [24]. The moderately high 1-butanol KP (3.3) shown by the diluent, OA, is in line with other reported values [3]. Abraham and Acree, Jr. [25] reported that the affinity between 1-butanol and OA affinity is mainly due to hydrophobic or dispersive interactions, but OA also shows greater hydrogen bond acidity interactions than [Ph3t][NTf2] or [OMA][NTf2]. The LM extractants’ KP increased with increasing equilibrium extraction temperature; generally DKP E2 for DT¼45 1C. Partitioning thermodynamics theory suggests that solute partitioning is the result of two factors influenced by temperature: the solute’s solubility in water and the molecular packing density of the organic phase [26]. It was shown that 1-butanol’s aqueous solubility actually decreases at increasing temperature at low range (25–50 1C) before it increases consistently at higher temperatures [27]. This means that increasing temperature actually facilitated 1-butanol partitioning to the RTIL phase. Although the decreased aqueous solubility trend applies only up to  50 1C, it is assumed that the resultant decrease in the RTIL phase’s molecular packing assisted the penetration of 1-butanol molecules; thus the increasing KP versus temperature behavior is maintained. Other studies of solute partitioning by RTILs have reported similar tendency [28]. 3.2. SILM-based pervaporation performances

3. Results and discussion 3.1. LM extractants0 1-butanol KP values The KP values for 1-butanol between the aqueous phase and the LM extractants were recorded at different temperatures (Fig. 3). At 25 1C, the LM extractants’ KP values were ranked: [Ph3t] [DCN] (7.4970.09) 4OA (3.32 70.18)4[OMA][NTf2] (1.447 0.07) 4[Ph3t][NTf2] (1.1070.16). The ionic liquid extractants’ 1-butanol affinities were more dependent on their anions, with [DCN] 4[NTf2], because of 1-butanol’s stronger hydrogen bonding with [DCN] than with [NTf2] [24]. Comparison of the 1-butanol KP values of [Ph3t][NTf2] and [Ph3t][DCN] shows that hydrophobic interactions were not as important as hydrogen bonding for 1-butanol partition. The [NTf2] anion is very hydrophobic due to its trifluoromethyl (–CF3) groups, though this did not induce as great a 1-butanol affinity as the [DCN] anion. [OMA][NTf2] and [Ph3t][NTf2] showed slightly different partitioning characteristic

3.2.1. Choice of RTIL The SILM performances were initially compared through pervaporation trials under common operating conditions (1.0 wt% 1-butanol feed concentration, 45 1C). The effects of relevant RTIL properties on the pervaporation performance parameters were also analyzed (Table 2 and Fig. 4). The PSI values indicate that the butanol separation performances of the pure RTIL-based SILMs ranked: [Ph3t][DCN]b[Ph3t][NTf2]4 [OMA][NTf2]. As expected, the 50/50 vol% [Ph3t][DCN]/[Ph3t][NTf2] blend (indicated as [Ph3t][NTf2/DCN]) showed a performance between those of its components. 1-Butanol flux exhibited highly nonlinear relationships with LM KP (positive) and water-saturated viscosity (negative) (Fig. 4a). Such nonlinearity is in agreement other studies; for example, Ferguson and Scovazzo’s results regarding gas diffusivity versus ionic liquid viscosity [9]. A high affinity to the permeating solute ensures SILMs to show high rates of sorption. However, LMs with high viscosity will exhibit low permeant diffusion rates. The

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Table 2 Effects of the RTILs used in SILMs on pervaporation performance. Pervaporation performance indicatorsc

RTIL and SILM properties

mdry

mwetb

LMC (cm3/g support) Fresh

91 83 123 114f

46d 42d 37e 39f

0.667 0.674 0.586 0.626

Viscosity, m (cP) RTIL as LM

[Ph3t][NTf2] [OMA][NTf2] [Ph3t][DCN] [Ph3t][NTf2/DCN]f

KP a

2.06 2.48 7.92 4.99f

(0.02) (0.01) (0.22) (0.11)

SILM WCAc, (1) Fresh

Used

76 90 58 59

79 (2) 92 (2) 71 (5) –

(3) (2) (2) (2)

J1  butanol (gm  2h  1)

Jwater (gm  2h  1)

BuOH in permeate (wt%)

Separation factor, b

PSI  10  3

29 (3.9) 22 (8.7) 126 (7.4) 45 (7.0)

48 (5.4) 60 (24.2) 172 (8.0) 72 (5.3)

37 26 42 38

63 37 77 57

4.8 (1.07) 2.9 (1.49) 22.5 (1.63) 6.6 (1.41)

(2.3) (3.1) (2.6) (2.0)

(8.4) (8.8) (5.6) (6.2)

a

KP at 45 1C interpolated from the KP vs. T curves in Fig. 1. Estimate of water-saturated viscosities based on Jacquemin et al.’s finding [29]. Average value (standard deviation); number of trials Z 3, at 1.0 wt% 1-butanol feed, 45 1C. d Relatively more hydrophobic. e Less hydrophobic (see Table 1 for solubility of water in RTIL data). f 50/50 vol% [Ph3t][NTf2]/[Ph3t][DCN] blend; properties estimated by averaging (for KP) and by mole fraction (x) based additive relation for mixture viscosity: ln m ¼ x1 ln m1 þx2 ln m2 [30]. b c

solution-diffusion model suggests that 1-butanol flux should be directly related to the LM’s solution capacity (i.e. partitioning or sorption) and inversely related to its viscosity. The SILMs were shown to follow the relationship since 1-butanol flux was positively correlated with KP/mwet (correlation coefficient, r ¼0.9549). The hydrophobicities of the freshly prepared SILMs made with pure RTILs were ranked: [OMA][NTf2]4[Ph3t][NTf2]4[Ph3t][DCN], in good agreement with the RTIL’s surface tension differences versus water (dwater ¼72 dyne/cm, Table 1). After use, the SILMs showed significantly increased water contact angles, confirming that the LM films on the surfaces of the support matrices diminished, since increased roughness and exposure of the support should increase water contact angle. The SILMs’ high hydrophobicity (i.e. water contact angle) did not necessarily ensure lower water permeation across them. For example, the [OMA][NTf2] SILM was significantly more hydrophobic than the [Ph3t][NTf2] SILM but showed greater water flux. Instead, the solubility of water in the RTILs, which were ranked: [Ph3t][DCN]4[OMA][NTf2]4[Ph3t][NTf2] (Table 1), were better correlated with water flux. Similarly, the SILMs’ high hydrophobicities were associated with low separation factors (Fig. 4b); LM KP had a stronger positive influence on separation factor. These relationships suggest that the RTILs’ solvation of 1-butanol (i.e. KP), which was more dependent on hydrogen bonding than on non-polar (i.e. hydrophobic) interactions, predominantly determined the SILMs’ selectivities. Comparison of the SILMs of this study to the other ionic liquidbased membranes recently reported in literature showed that with proper selection of membrane components, SILMs could potentially yield comparable or better results. Matsumoto et al. [31] have shown that at similar condition (31 mm membrane thickness, 5 g/L feed, 25 1C), a polymer inclusion membrane (PIM) composed of trioctylammonium chloride (Aliquat 336 or [OMA][Cl]) RTIL and polyvinyl chloride yielded 1-butanol flux value (26 g  2 h  1) which was within the range shown in Table 2. However, the separation factor of the PIM for 1-butanol was considerably lower (b ¼4.5); a direct consequence of high water flux. As a halide anion-based RTIL, Aliquat 336 is expected to have higher affinity to water than [OMA][NTf2]. Thus, the result described by Matsumoto et al. corroborates with this work’s finding on the positive influence of the solubility of water in the SILM’s selective component on water permeation.

3.2.2. Effects of the feed’s 1-butanol concentration Pervaporation using the three SILMs was explored at 60 1C using varying concentrations of 1-butanol in the feed (Fig. 5).

Their recoveries were comparable to previously reported values [32]. The [Ph3t][DCN] SILM exhibited an exceptionally high 1-butanol flux. 1-Butanol flux was almost linearly related to concentration (Fig. 5a). Water flux was either largely unchanged (in the [Ph3t][NTf2] and [OMA][NTf2] SILMs) or decreased (in the [Ph3t][DCN] SILM) with increasing 1-butanol feed concentration. The permeant fluxes were attributed to the linear increase of 1-butanol activity with increasing concentration while water activities are relatively constant in dilute aqueous solutions [33]. The linear relationship between 1-butanol flux and feed concentration indicates constant 1-butanol permeability [34]. The positive dependence of 1-butanol flux on the feed concentration was most evident in the [Ph3t][DCN] SILM, then the [Ph3t][NTf2] SILM, followed closely by the [OMA][NTf2] SILM. This trend is in general accordance with the respective RTILs’ 1-butanol separation and transport properties. The decreasing water flux shown by the [Ph3t][DCN] SILM with increasing feed concentration was not explicable by constant water activity and was hence further explored and discussed next. The described 1-butanol and water flux behaviors led to separation factors that consistently increased with increasing feed concentration (Fig. 5b). This is unusual for pervaporation membranes. As a typical example, Fouad and Feng studied 1-butanol pervaporation from dilute solutions using a poly(etherblock-amide) membrane and reported a consistent decrease and leveling off of separation factor at higher feed concentrations [34]. This was attributed to coupling effects between the permeating species (1-butanol and water), which are both capable of polar interactions [34]. In comparison, the results of this study indicate that there was no significant coupling in the SILMs.

3.2.3. Effects of feed temperature Pervaporation was investigated at 35–70 1C. Temperature is an important parameter as it affects both sorption and diffusion. It is especially important to liquid membranes as liquids are generally more responsive to the effects of temperature than solid polymers or inorganic materials. As the [Ph3t][NTf2] membrane performed better than the [OMA][ NTf2] membrane, further investigations focused only on [Ph3t][NTf2] and [Ph3t][DCN] (Fig. 6). 1-Butanol and water fluxes consistently increased with increasing temperature. Component flux increased with increasing temperature when the feed’s 1-butanol concentration was constant due increased vapor pressure driving force. The SILMs’ performances deviate from those of polymeric membranes in that there was a late onset of the trade-off between flux and separation

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219

Fig. 5. Effects of feed’s 1-butanol concentration on (a) fluxes of 1-butanol and water, and (b) 1-butanol permeate concentration and separation factor at 60 1C.

Fig. 4. Pervaporation performance with respect to SILM/LM properties: (a) J1  butanol versus LM KP (closed markers) and versus LM water-saturated viscosity (open markers); (b) separation factor b versus LM KP (closed markers) and versus fresh SILM hydrophobicity or water contact angle (open markers).

the liquid-phase sorption coefficient (KLi or KP), the concentration of species i in the liquid phase (Cio), and the partial vapor pressure (p) of species i in equilibrium with the liquid phase [36] K Gi ¼

factor. The separation factors of both SILMs consistently improved as temperature increased from 35 to 60 1C, before declining at 70 1C. Although not common, similar lacks of trades-off have been reported in dense membranes that show minimal swelling or plasticization [35]. The data in Fig. 6 were converted to component permeances by Eq. (5) and SILM selectivity by Eq. (6) (Fig. 7). This conversion resulted in normalized variations in the performance data due to the changes in 1-butanol and water vapor pressure, supporting the aforementioned interpretations of the effects of the feed’s increased 1-butanol concentration and temperature on the permeants’ fluxes through increasing their partial pressure driving forces. As expected, permeances of both 1-butanol and water decreased with increasing temperature, at a given feed concentration. Since diffusivity generally increased with increasing temperature (Section 3.2.4), so sorption had decreased and exerted greater effect on transport. Baker et al. reported that the gas-phase sorption coefficient (KG i ) is related to

C io K Li C io K Li ¼ pi gio xio psat io

ð12Þ

It is suggested that for a given 1-butanol feed concentration, increasing temperature would lead to lesser increases of KLi (Fig. 3) and activity coefficient, g, than the increase of psat io , thereby effectively decreasing KG i . The SILMs showed significantly higher selectivities to 1-butanol (Fig. 7c and d) than pure PDMS (aPDMS 1  BuOH/water E 1.8) but slightly lower than that shown by a PDMS-zeolite mixed matrix membrane (aMMM 1  BuOH/water E6) reported by Vane et al. [37]. Consistent and positive influence on selectivities by the feed’s 1-butanol concentration was evident, indicating that the ‘‘crowding’’ (saturation) effects common in zeolite filled polymeric membranes [18] did not occur in the SILMs. The [Ph3t][NTf2] SILM’s selectivity was enhanced due to the modest improvement in 1-butanol permeance with increased feed 1-butanol concentration. That of the [Ph3t][DCN] SILM increased due to the suppression of water permeance with increasing feed 1-butanol concentration. Selectivities greater than unity indicate that the

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Fig. 6. Effects of temperature on the pervaporation of 1-butanol by (a and b) [Ph3t][NTf2] and (c and d) [Ph3t][DCN] SILMs at varying feed concentrations.

Fig. 7. Permeances of (a) 1-butanol and (b) water; (c and d) selectivity data using SILMs based on [Ph3t][NTf2] (solid trendlines) and [Ph3t][DCN] (dashed trendlines) (1 gpu ¼1  10  6 cm3(STP)/cm2 s cm Hg).

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SILMs have achieved separation of 1-butanol–water solutions at a significantly higher degree than a simple evaporation (i.e., due to vapor–liquid equilibrium) would produce [18]. 3.2.4. Diffusion coefficients The transport mechanisms during SILM-based pervaporation were investigated by estimating diffusion coefficients from Eq. (11) (Fig. 8). 1-Butanol and water’s flux data fitted well to Eq. (11), implying that the diffusion coefficients were constant across the SILMs and did not vary with concentration. This is also in accordance with the fundamental Stokes–Einstein equation where diffusion of a solute in liquids depends only on the solute molecular size, liquid viscosity and temperature. For 1-butanol, Di was calculated from the slopes. For water, the equation Dj ¼

t

Jl j

e r ln 1jo j j



ð13Þ

was applied to each data point and then averaged, as there was practically zero slope (constant water flux across the variation of ln(1  j0j )) when Eq. (11) was used to fit the water flux data. The diffusivity values of 1-butanol, computed from the flux data (Eq. (11)), were in the ranges of 4–9  10–11 m2/s for the [Ph3t][NTf2] SILM and 2.5–8  10  11 m2/s for the [Ph3t][DCN] SILM. The smaller water molecules showed higher diffusivities: 2–7  10  10 m2/s and 7  10–11 2  10  10 m2/s for the two SILMs.

221

Although both membranes showed similar ranges of 1-butanol diffusivities, the [Ph3t][DCN] SILM exhibited a 1-butanol flux approximately twice that of the [Ph3t][NTf2] SILM (Fig. 6), further suggesting that 1-butanol permeation was governed by sorption. The diffusivity of water increased at a greater rate than that of 1-butanol, particularly at higher temperatures (60–70 1C). Water-saturated ionic liquids have shown phase separation (water aggregations) at the microscopic level, which can greatly increase water diffusivity [38] and disrupt the molecular diffusion mechanism for water across a SILM [39]. The formation of water micro-aggregates is expected to be more pronounced in hygroscopic RTILs and at higher temperatures, where water solubility in the ionic liquids is generally increased. Unexpectedly, [Ph3t] [NTf2] showed higher water diffusivities than the more hygroscopic [Ph3t][DCN], possibly because of the obstruction of water diffusion by the presence of sorbed 1-butanol at elevated concentrations due to [Ph3t][DCN]’s high KP. Rollet et al. [38] reported that phase-separated water in an RTIL forms a ‘‘porous’’ microscale network where the pore walls are richer in waterinteracting anions. In the presence of 1-butanol, most of the water molecules at the pore walls are likely to be displaced by the alcohol molecules, restricting the network in which water can diffuse. 3.2.5. Activation energy analysis The temperature dependencies of component flux, permeance and diffusivity according to Arrhenius-type relationships [35]   Ei Ji ¼ J o,i exp ð14Þ RT   Pi Epi ¼ Po,i exp l RT

ð15Þ

  Edi Di ¼ Do,i exp RT

ð16Þ

were verified and results are summarized in Table 3. In Eqs. (14)–(16) where Jo,i, Po,i, and Do,i are pre-exponential parameters and Ei, Epi, and Edi, are the activation energies of the permeation flux, membrane permeance, and diffusion, respectively. R is the ideal gas constant and T the absolute temperature (K). The molar heats of vaporization (DHvi), mixture (DHmi), and solution or dissolution (DHsi) were estimated by

Fig. 8. Diffusion coefficients of 1-butanol and water with respect to temperature.

DHvi ¼ Ei Epi

ð17Þ

DHmi ¼ Ei Edi

ð18Þ

DHsi ¼ DHmi DHvi

ð19Þ

Table 3 Activation energies, heat of mixture, heat of vaporization and heat of solution of butanol and water for SILM pervaporation at 1.0% butanol feed concentration. (KJ/mol)

DHmi Ei Edi DHvi DHsi Epi

a b c

[Ph3t][NTf2] SILM

[Ph3t][DCN] SILM

1-Butanol

R2a

Water

R2a

1-Butanol

R2a

Water

R2a

29.7 48.2 18.5 57.5  27.8  9.3

– 0.9972 0.5655 – – 0.9046

14.8 45.4 30.6 46.0  31.2  0.6b  3.6c

– 0.9913 0.9842 – – 0.0416b 0.8001c

12.1 41.2 29.2 58.6  46.6  17.4

– 0.9982 0.9578 – – 0.9750

7.5 38.4 30.9 44.2  36.8  5.9b  11.0c

– 0.9871 0.9980 – – 0.5983b 0.9772c

Coefficient of determination. Permeance data at all temperatures included in the analysis. Permeance data at 70 1C excluded from the analysis.

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Fig. 9. SILMs’ sustained pervaporation performances (a, b and c) and concentrations of RTIL in feed (d) corresponding to (a) and (b), all at 15 g/L 1-butanol feed concentration, 60 1C.

Higher activation energies imply greater sensitivity to temperature changes [35]. Both SILMs showed consistently higher E,BuOH than E,water, indicating that 1-butanol permeation increased at a higher rate than water permeation with increasing temperature. Such behavior was attributed to the RTILs’ organophilic nature. The Ep for 1-butanol in the RTILs was consistently negative, indicating that the vapor sorption contribution (DHs) was more important than that from diffusion (Ed) [35]. The predominance of sorption selectivity in the RTILs is supported by the decreasing 1-butanol permeance with respect to temperature. [Ph3t][DCN] showed lower heats of mixture (DHm), confirming its stronger tendency for molecular interactions with the permeants than [Ph3t][NTf2]. The peculiarities on RTIL-water-1-butanol interactions are also reflected in the activation energies. The SILMs’ Epwater values were adequately described by Arrhenius-type linear relationships at lower temperatures (35–60 1C) but not at the highest tested temperature (70 1C). This result signifies that water permeation could not be adequately described by temperature alone and suggests a confirmation of the microphase separation phenomenon in the RTILs. It is deduced that the microenvironments’ influence on water transport become more important at higher temperatures. The SILMs’ Ep1  butanol values closely followed Arrhenius relationships, highlighting their independence from water transport and consistent with an absence of coupling effects. Although [Ph3t][DCN] was more hygroscopic than [Ph3t][NTf2], their respective Ed values for water diffusion were approximately equal due to the obstructing effects of 1-butanol on water diffusion in the [Ph3t][DCN] SILM. The [Ph3t][NTf2] SILM’s 1-butanol diffusivity data did not closely fit an Arrhenius

relationship (low R2) because 1-butanol diffusivity was approximately stable at 45–70 1C (Fig. 8). [Ph3t][NTf2]‘s relatively low affinity for 1-butanol suggests that molecular obstruction by 1-butanol was not strong, allowing more efficient water diffusion through the water ‘‘pore’’ network. The RTILs’ levels of water saturation were increased at higher temperatures, making the water ‘‘pore’’ networks less tortuous [38]. 3.3. SILM intrinsic stability and LM blending The [Ph3t][NTf2] and [Ph3t][DCN] SILMs’ intrinsic stabilities were assessed through sustained pervaporation for 90 h at 60 1C using a 1.5% 1-butanol feed. Although the duration of testing was much shorter than in other studies [4,37] and only one type of support matrix has been used, the [Ph3t][NTf2] SILM already showed high prospect for intrinsic stability (Fig. 9a). The potentially stable performance of [Ph3t][NTf2] SILM was attributed to the retention of immobilized RTIL in the support pores (Fig. 10), which was achieved through the combined effects of the RTIL’s hydrophobicity, viscosity, surface tension, and compatibility with the particular support matrix. [Ph3t][NTf2] showed the lowest capacity for water saturation (Table 1) and thus was not susceptible to emulsification. The concentration of RTIL dissolved in the feed solution was largely stable (Fig. 9d), suggesting that the RTIL in the feed was only from the dissolution of excess RTIL coating from the exposed SILM’s surface. The high separation performance of the [Ph3t][DCN] SILM was coupled with instability (Fig. 9b). Performance deteriorated as early as around 10 h when water permeance began to increase monotonically and 1-butanol permeance started to decrease.

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suggested that water transport was governed by the presence of water microenvironments in the liquid membranes, which became more prevalent at higher temperatures. Performance parameters for permeance and membrane selectivity confirmed that sorption governed permeation and that the SILMs were consistently selective to 1-butanol. The SILMs’ selectivities increased with the feed’s increasing 1-butanol concentration possibly due to the capacity of 1-butanol to block water sorption and diffusion. The [Ph3t][NTf2] SILM demonstrated good potential for intrinsic stability during sustained pervaporation testing but further verification is required using alternative support matrices. The high separation performance of the [Ph3t][DCN] SILM was offset by its instability. Dilution with OA temporarily enhanced the [Ph3t][NTf2] SILM’s separation performance by increasing KP and decreasing viscosity—until the OA was gradually leached from the SILM.

Acknowledgment

Fig. 10. FESEM images (5000  ) of pristine Celgards 2400 support (inset) and [Ph3t][NTf2]-based SILM used for sustained pervaporation testing.

At around 16 h, the SILM had become more selective to water than to 1-butanol. The spent SILM appeared opaque and white, similar to the pristine support matrix, suggesting that the RTIL was displaced from the support’s pores. This was supported by the continuously rising levels of dissolved RTIL in the feed (Fig. 9d). LM blending to fine tune pervaporation performance was tested in a [Ph3t][NTf2] and OA (50/50 vol.%) mixture as [Ph3t][DCN] was shown to be unstable. The blended SILM initially showed improved 1-butanol selectivity through a significant rise (10–16%) in 1-butanol permeance over the pure LMs (Fig. 9c), which was due to an enhanced 1-butanol KP [11] and a lower viscosity that resulted from blending. However, OA appeared to be preferentially leached from the blended SILM as the membrane’s separation performance gradually declined towards that of the pure [Ph3t][NTf2] membrane. Favre et al. reported similar problems in maintaining OA impregnated in PDMS membranes [33]. The solubility or emulsification of OA in water is greatly enhanced in the presence of even very small amounts of 1-butanol [3].

4. Conclusions Simple SILMs’ performances and intrinsic stabilities during the recovery of 1-butanol from dilute solutions were tested using hydrophobic ammonium- and phosphonium-based RTILs as liquid membranes. 1-Butanol flux was strongly related to the RTIL’s KP for 1-butanol and was also proportionally dependent on the feed’s concentration of 1-butanol. Coupling between 1-butanol and water permeation was not observed and the RTILs’ water saturation capacities strongly influenced water permeation. Separation factor was positively correlated with the RTIL’s KP for 1-butanol and negatively correlated with SILM’s hydrophobicity, confirming the importance of hydrogen bonding, rather than non-polar or hydrophobic interactions, in the RTILs’ 1-butanol sorption. The late onset of the trade-off between temperature and separation factor highlights the potential advantages of SILMs over conventional membranes. Diffusivity and activation energy analyses

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